Expression of the approximately 70 herpes simplex virus type 1 (HSV-1) genes is regulated primarily at the level of mRNA transcription, and can be conveniently divided into three phases based on the temporal order of mRNA synthesis. Immediate-early genes are expressed soon after virus infection, and their gene products are required for the expression of early genes, many of whose gene products are involved in viral DNA replication. Expression of viral late genes requires viral DNA replication and functional immediate-early gene products. Understanding the regulation of HSV gene expression is necessary for the development and evaluation of successful vaccines, particularly live virus vaccines. In addition, the development of HSV as a gene therapy vector requires an understanding of the regulation of foreign gene expression in HSV vectors. One part of this research project is designed to study the temporal expression of herpes simplex type 1 late genes, those genes expressed only after viral DNA replication. Most of the products of these genes are structural proteins of the virion and include at least 15 glycoproteins and 7 different capsid proteins. We have shown that late HSV promoters require a TATA element and an initiator element at the start of transcription for maximal activity. These elements can be replaced by similar eukaryotic elements. This suggests that late promoters are composed of eukaryotic regulatory elements and that the arrangement and composition of promoter elements determines when a gene is expressed during viral infection. A second part of this work investigates the expression of foreign genes in herpes simplex vectors. We have expressed the HIV Gag gene in a replication-incompetent HSV vector, and shown that high levels of expression can be obtained using the immediate-early promoter from the ICP0 gene. This promoter may be useful for foreign gene expression in such vectors since it is overexpressed in the absence of the immediate-early protein ICP4. The third part of this research project is designed to evaluate various HSV gene products as potential components of subunit vaccines. Expression plasmids have been constructed to express HSV glycoproteins D, C, or E. Direct injection of these plasmids into mice elicits an antibody response and protects the mice from subsequent lethal challenge. We are currently comparing the relative protective effects of the different expression plasmids, and evaluating various combinations for optimal protection against HSV infection.